For autonomous vehicles to navigate properly, it is essential to have accurate, reliable, and continuous positioning available. Precise positioning algorithms based on Real-Time Kinematics (RTK) using Global Navigation Satellite System (GNSS) are widely used for this purpose and can provide a centimeter level accuracy in open sky environments. However, the positioning availability and accuracy are subject to degradation due to the different environmental aspects and GNSS satellite availability. AUTO, previously Coursa Drive, is a multi-sensor integration system, designed to help alleviate these problems by providing a seamless navigation solution, regardless of the environment. AUTO integrates an RTK GNSS with an Inertial Navigation System (INS) to correct for sensor errors and calibrate the INS when an accurate RTK GNSS solution is available. Additional sensors such as an odometer and barometer are added to assist in reducing the error growth during outage intervals. The integrated solution provides a continuous, reliable, and accurate solution consisting of position, velocity, attitude and heading together with all their accuracy measures to the autonomous vehicle system. Decimeter level precision can be achieved for short periods when the GNSS or perception systems are unavailable. AUTO has been tested in different environments, with widely different weather and temperature characteristics to assure robustness and reliability across various conditions. The testing included a variety of simulated and natural GNSS outages to provide key performance indices (KPI) in those outages. For high-speed scenarios on highways needed for level 2 and level 3 of autonomous vehicles, and during full blockage of GNSS signals in both natural or simulated outages, AUTO enhances the sensor-only navigation solution and vehicle positioning to <0.2% error of the traveled distance.